The present invention relates in general to controlling switching transients for power switching transistors, and, more specifically, to temperature-compensated gate drive signals for power converters of a type used in electrified vehicles.
Electrified vehicles, such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs), use inverter-driven electric machines to provide traction torque. A typical electric drive system includes a DC power source (such as a battery pack or a fuel cell) coupled by contactor switches to a variable voltage converter (VVC) to regulate a main bus voltage across a main DC link capacitor. An inverter is connected between the main buses for the DC link and a traction motor in order to convert the DC power to an AC power that is coupled to the windings of the motor to propel the vehicle.
The inverter includes transistor switching devices (such as insulated gate bipolar transistors, or IGBTs) connected in a bridge configuration including a plurality of phase legs. A typical configuration includes a three-phase motor driven by an inverter with three phase legs. An electronic controller turns the switches on and off in order to invert a DC voltage from the bus to an AC voltage applied to the motor. The inverter is controlled in response to various sensed conditions including the rotational position of the electric machine and the current flow in each of the phases.
The inverter for the motor may preferably pulse-width modulate the DC link voltage in order to deliver an approximation of a sinusoidal current output to drive the motor at a desired speed and torque. Pulse Width Modulation (PWM) control signals are applied to drive the gates of the IGBTs in order to turn them on and off as necessary. In an idealized form, the gate drive control signals are square wave signals that alternate each power switching device (e.g., IGBT) between a fully off and a fully on (saturated) state. During turn off and turn on, it takes time for the device to respond to the change in the gate drive signal. For example, after the gate drive signal transitions from a turn-off state to a turn-on state, conduction through the device output transitions from zero current flow to a maximum current flow within a few microseconds.
The optimal switching speed of a power semiconductor transistor device such as an IGBT is a tradeoff between high stresses which could destroy the device at very fast switching speeds and reduced efficiency and increase power losses at slower switching speeds. Drive circuitry for the device may be configured to energized the gate terminal of the transistor with a time varying control signal that follows a trajectory to optimized the switching speed. With changes in the temperature of the transistor, however, the switching speed also changes in response to certain parameters of the transistor including internal gate resistance, threshold voltage, and trans-conductance. Generally, as temperature increases the switching speed decreases, so that switching losses increases; as temperature decreases, the voltage and current stress increases, so that the reliability decreases. To avoid increased stress and reduced efficiency, it becomes necessary to compensate for the temperature-induced parameter changes.
A conventional design criterion for selecting the best gate control signal trajectory or slope (e.g., as determined by the gate resistance or similar control parameters) is to optimize the switching performance at the worst case—which occurs at the lowest operating temperature. Therefore, as temperature increases and the device switching speed is accordingly decreased, and there is a need adjust the control parameter in a way that tends to increase the switching speed. The switching speed has typically been increased by increasing the magnitude and/or slope of the current being supplied to the gate by the gate drive signal in proportion to the temperature increase. For example, the gate current can be varied directly by using a controllable current source for the gate driver, or the gate current can be manipulated indirectly by increasing the gate voltage or decreasing the gate resistance. The parameters have been adjusted 1) using a closed-loop control system based on a measured temperature, and 2) automatically by incorporating a negative-temperature coefficient (NTC) resistance connected to the gate, for example. However, it has been found that not all aspects of the switching transient need to be adjusted by temperature variations with controlled current source. Thus, even better optimization is possible beyond the blanket modifications to the gate drive signals used in the prior art.